A study on pyrolysis of wood of different sizes at various temperatures and pressures

In this work, three different particle sizes (0.5 ∼ 1 mm, 2 mm and 5 mm) of pine wood were pyrolyzed at the temperature range between 350 and 800 °C and pressures between 1 and 8 bar in a pressurised fixed bed reactor. A monolayer of different particle sizes of biomass samples was also pyrolysed at various pressures to assess by comparison the effect of intra-particle vs inter-particle secondary reactions on char formation. The relatively small particle and reactor size as well as the slow heating, enable assessing the effects of particle size and pressure in the absence of heat transfer limitations. The liquid and solid products were characterized by various techniques, including gel permeation chromatography (GPC), gas chromatography with mass spectrometry detection (GC–MS), elemental analyser and Raman spectroscopy. The combustion reactivity of chars obtained at various conditions was tested by thermogravimetric analysis (TGA). Higher temperatures promote tar and gas yields at the expense of solid residues. The chars produced at higher temperatures show greater aromaticity and lower combustion reactivity. The larger particle size can decrease tar formation at 350 °C, while smaller particle sizes decrease tar yield through the inter-particle secondary reaction within the hot char bed at 500 °C and above. For larger particle sizes, a stronger intra-particle secondary process can make the chars more ordered, thus leading to a lower combustion reactivity. Higher pressure can hinder the vaporization of fragments and promote the formation of secondary chars, thus less tar and more char were obtained. Chars produced at higher pressures show a lower combustion reactivity due to the higher aromaticity of secondary chars. Therefore, higher temperature, larger particle size and higher pressure can all produce more ordered chars with lower combustion reactivity.